The present invention relates to a method for forming a film of oxidized metal or, more particularly, to a method for forming a film of oxidized metal exhibiting greatly increased electric insulation.
As is known, application fields of oxidized metal films are rapidly expanding in recent years with variety including, for example, insulating films or orientation-controlling films in liquid-crystal display units, protecting films on ceramics and metals, insulating films on semiconductor devices and so on. In a liquid-crystal display unit, in particular, an insulating substrate of, for example, glass is provided on the surface with a patterned transparent electroconductive film to serve as the electrodes on which an oxidized metal film is formed to form an electrode substrate and a pair of such electrode substrates each having an oxidized metal film are assembled to face each other with spacers therebetween around the peripheris to form a cell to be filled with a liquid-crystal material.
To give an example in more detail, a glass substrate coated with a surface film of silicon dioxide SiO.sub.2, is first provided with a patterned transparent electroconductive film, such as a so-called ITO film composed of oxides of indium and tin, formed thereon as the electrodes and then coated over the whole surface thereof with an insulating film of an oxidized metal.
Such an oxidized metal film is required to have a characteristic that the oxidized metal film per se is electrically highly insulating in addition to the requirements of high adhesion to the substrate and transparent electroconductive film and uniformity of the oxidized metal film per se as a matter of course. Along with the increasing demand in recent years for higher and higher precision in liquid-crystal display units and finer and finer precision of working in transparent electroconductive films, it is a trend in the design of liquid-crystal display units that the distance between adjacent patterned electrodes and the gap space between the oppositely facing electrodes are extremely small. Accordingly, the oxidized metal film formed on the transparent electroconductive film is required to be extremely highly insulating in order to prevent any malfunctioning otherwise possibly taking place between the electrodes.
A method undertaken in the prior art for increasing the electrical insulation of an oxidized metal film is that a coating film for forming an oxidized metal film is formed on the surface of a substrate by using a coating solution for forming an oxidized metal film followed by a heat treatment at a high temperature of at least 400.degree. C., preferably, at least 500 .degree. C. Although this method is very effective for increasing the electrical insulation of the oxidized metal film per se, it is not always a practically advantageous method because the oxidized metal film is used as formed on an electrode such as a transparent electroconductive film as is mentioned above so that the heat treatment at a high temperature sometimes badly affects the transparent electroconductive film. When a coating film for forming an oxidized metal film is formed over the whole surface of a patterned ITO film and then subjected to a conventional heat treatment to obtain a highly insulating oxidized metal film, a serious drawback is unavoidably caused that the characteristics of the ITO film are affected or, for example, the resistance of the ITO film is disadvantageously increased resulting in a decrease in the performance of the electrode even though the oxidized metal film can be highly insulating. This situation leads to a need of decreasing the temperature in the heat treatment of a coating film for forming an oxidized metal film.
In view of the above described problems, it is eagerly desired to develop a method to increase the electrical insulation of an oxidized metal film useful as an insulating material.